# LOFAR observations of 4C+19.44. On the discovery of low frequency spectral curvature in relativistic jet knots

## TL;DR

This study uses LOFAR to observe the quasar jet 4C+19.44, revealing low-frequency spectral curvature in jet knots, suggesting stochastic acceleration processes influence particle energy distributions.

## Contribution

First LOFAR observations of 4C+19.44's jet reveal spectral curvature at low frequencies, supporting stochastic acceleration as a key process in quasar jet knots.

## Key findings

- Spectral curvature observed at hundreds of MHz in jet knots.
- Radio flux densities are below extrapolated GHz spectra.
- Results support stochastic acceleration mechanisms in jet particles.

## Abstract

We present the first LOFAR observations of the radio jet in the quasar 4C+19.44 (a.k.a. PKS 1354+19) obtained with the long baselines. The achieved resolution is very well matched to that of archival Jansky Very Large Array (JVLA) observations at higher radio frequencies as well as the archival X-ray images obtained with {\it Chandra}. We found that, for several knots along the jet, the radio flux densities measured at hundreds of MHz lie well below the values estimated by extrapolating the GHz spectra. This clearly indicates the presence of spectral curvature. Radio spectral curvature has been already observed in different source classes and/or extended radio structures and it has been often interpreted as due to intrinsic processes, as a curved particle energy distribution, rather than absorption mechanisms ({ Razin-Tsytovich} effect, free-free or synchrotron self absorption to name a few). Here we discuss our results according to the scenario where particles undergo stochastic acceleration mechanisms also in quasar jet knots.